MAKE SURE THAT YOU HAVE SET YOUR "SOFTWARE SOURCES SETTINGS" FIRST BEFORE TRYING TO ADD PACKAGES TO YOUR SYSTEM, SEE THE UPDATE GUIDE FOR FURTHER INFO!:
Any packages/programs that you wish to download and install can be done using the (Ubuntu) Software Center, Synaptic package manager or using the terminal (Ctrl + Alt + t) if you know the commands.
An example of a terminal install package command:
sudo apt-get install synaptic
(replace synaptic with the name of the package you wish to install!)
Any 3rd party packages that you might want to install should end with ".deb" or "all.deb" and should downloaded from a known and verified source/location, in other words; a place that you know is safe to download from, a good example would be adobe.com or the google earth website.
It is also important to note that you should make sure that you download the correct package for your PC, either 32 bit; or 64 bit, packages ending "all.deb" work with both systems!!.
There are also "tar" files which you can always use providing you follow the instructions given with each package (AFTER YOU HAVE EXTRACTED THE FILES) which are normally found in a "Read Me" file!!. (safety rule applies here too!)
Do not try and use "rpm" packages if you possibly can as they are designed for another type of packaging system and may well cause damage!. (Redhat Package Manager) See below about RPM conversion using "Alien".
Download deb packages from here:
For installing single packages, install the app "Gdebi":
sudo apt-get install gdebi
To convert RPM packages to DEB packages, use "Alien":